1. Field of the Invention
The present invention relates to an exposure method and a method of manufacturing a semiconductor device and, more particularly, to a technique of reducing the influence of aberration in a lithography process.
2. Description of the Related Art
Recently, with a reduction in the size of a circuit pattern, the influence of aberration of a projection lens in an exposure apparatus poses a problem.
As the aberration of the projection lens decreases, optophysical treatment is required. For this reason, the aberration of an optical system is converted into wave aberration. As a method of expressing wave aberration as a function of pupil coordinate, a method using the Zernike polynomial is widely used. The first to 16th terms of this Zernike polynomial are represented as follows:                Z1: 1        Z2: rcos θ        Z3: rsin θ        Z4: 2r2−1        Z5: r2 cos 2θ        Z6: r2 sin 2θ        Z7: (3r2−2r)cos θ        Z8: (3r3−2r)sin θ        Z9: 6r4−6r2+1        Z10: r3 cos 3θ        Z11: r3 sin3θ        Z12: (4r4−3r2)cos 2θ        Z13: (4r4−3r2)sin 2θ        Z14: (10r5−12r3+3r)cos θ        Z15: (10r5−12r3+3r)sin θ        Z16: 20r6−30r4+12r2−1        
In this case, Z10 and Z11 are generally called 3θ aberration, which cause an increase in the asymmetry of a pattern and a decrease in DOF (Depth Of Focus). This problem will be described below with reference to FIGS. 12 and 13.
FIG. 12 schematically shows the distribution of Z10 on a pupil plane. The phase advance reaches its peak at 0°, 120°, and 240°, and the phase delay reaches its peak at 60°, 180°, and 300°, with reference to the forward direction of the X-axis.
Consider a case wherein diffraction light is produced in the X-axis direction by a pattern formed on a reticle. In this case, as shown in FIG. 13, zero order light and first order diffraction light on the wafer differ in their imaging positions. For this reason, the light intensity distribution on the wafer becomes asymmetrical. As a consequence, an asymmetrical pattern is projected on the wafer, and no desired pattern can be obtained.
Conventionally, the above problem is solved by adjusting a lens. With regard to 3θ aberration, it is very difficult to perform lens adjustment. It is therefore very difficult to solve the problem that the asymmetrical pattern is projected on a wafer.
When a checkered flag pattern is formed on a reticle, in particular, the influence of 3θ aberration increases owing to the intensity distribution of diffraction light on a pupil plane. For this reason, the problem that the asymmetrical pattern is projected on a wafer becomes more serious.
As described above, with a reduction in the size of a circuit pattern, it becomes more difficult to obtain a desired pattern because of the influence of the aberration of a projection lens. It is especially difficult to reduce the influence of 3θ aberration. Demands have therefore arisen for an exposure method which can project a desired pattern with high precision.